1. Field of the Invention
The present invention relates to an image detector in which a large number of pixels having a thin film transistor is two-dimensionally arrayed and specifically relates to a structure of the pixel thereof.
2. Description of the Related Art
A FPD (Flat Panel Detector) in which an X-ray sensitive layer is disposed on a TFT active matrix substrate and is capable of converting X-ray information directly into digital data, has been put into practice lately. The FPD has merit that enables one to instantly confirm images and even video images as compared to a prior art imaging plate. Therefore, it is spreading rapidly.
At first, radiation image detector using the FPD will be explained with reference to FIG. 20.
The radiation image detector has been constructed by forming a semiconductor film 6, having electromagnetic wave conductivity on an active matrix substrate 10 in which charge collecting electrodes 11 are disposed in array, and by forming a bias electrode 7 on the semiconductor film 6. Further, the upper electrode 7 is connected to a high voltage source.
The semiconductor film 6 is an amorphous selenium (a-Se) film of 100 to 1000 μm including selenium as a main component, and generates electric charges within the film when it is irradiated by X-rays. A TFT switch 4 and a charge storage capacitor 5 are provided in the vicinity of the charge collecting electrode 11 disposed in array on the active matrix substrate 10 and a drain electrode of the TFT switch 4 is connected with one electrode of the charge storage capacitor 5. Another electrode of the charge storage capacitor 5 is connected with a storage capacitor line 102. A scan line 101 is connected to a gate electrode of the TFT switch 4, and a data line 3 is connected to a source electrode. A signal detector (amplifier) 105 is connected to a terminal end of the data line 3 (see Japanese Patent Application Laid-open No. 11-190774 and 2001-135809 for example).
Next, a principle of operation of the above mentioned radiation image detector will be explained.
When the X-rays are irradiated from the upper part in FIG. 20, the semiconductor film 6 generates electric charges therein. Among the generated charges, positive holes are collected to the charge collecting electrode 11 by electric potential difference between the bias electrode 7 and the charge collecting electrode 11 and are stored in the charge storage capacitor 5 electrically connected with the charge collecting electrode 11. Because the semiconductor film 6 generates a different amount of electric charges corresponding to a dosage of X-rays, charges corresponding to image information carried by the X-rays are stored in the charge storage capacitor 5 of each pixel. After that, signals for turning the TFT switch 4 ON are sequentially added through the scan line 101 to take out the charges stored in each of the charge storage capacitors 5 via the data line 3. Then it becomes possible to read the image information by detecting an amount of charges of each pixel by the signal detector 105.
Next, a structure of the pixel when the TFT active matrix substrate is manufactured by using a technology for manufacturing a general liquid crystal panel or the like will be explained. FIG. 21 is a section view showing a structure of one pixel unit of the radiation image detector and FIG. 22 is a plan view thereof. FIG. 21 is a section view taken along a line 21-21 in FIG. 22.
As shown in FIG. 21 and FIG. 22, the radiation image detector has a gate electrode 2, the scan line 101, a storage capacitor lower electrode 14 and a storage capacitor line 102 on a glass substrate 1. Then, a gate insulating film 15 is provided on the gate electrode 2, the scan line 101, the storage capacitor lower electrode 14 and the storage capacitor line 102. A semiconductor layer 8 is formed on the gate electrode 2 through an intermediary of the gate insulating film 15. Then, source and drain electrodes 9 and 13 are formed on the semiconductor layer 8. A storage capacitor upper electrode 18 is deposited on a layer composing the charge storage capacitor 5. Then, a data line 3 is provided in the same metal layer with the source electrode 9, the drain electrode 13 and the storage capacitor upper electrode 18. Then, an insulation protecting film 17 is disposed above the data line 3, the storage capacitor upper electrode 18, the source and drain electrodes 9 and 13.
Further, an interlayer insulating film 12 is provided on the insulation protecting film 17. The charge collecting electrode 11 is provided on the interlayer insulating film 12, i.e., at the uppermost layer of the active matrix substrate 10. The charge collecting electrode 11 is connected with the TFT switch 4 via the storage capacitor upper electrode 18 and the drain electrode 13. Furthermore, the data line 3 crosses with the scan line 101 and the storage capacitor line 102 via the gate insulating film 15. Then, the semiconductor film 6 and the bias electrode 7 are formed on the active matrix substrate 10.
In a radiation image detector configured as above, there are two important problems which need to be addressed, these being the drop in production yield of the TFT array and deterioration of the image quality of detected images.
Explanation will first be given of the problem of the production yield of the TFT array.
Defects during production of the TFT array may be divided into the categories of line defects and point defects. Of these, correction of the point defects is possible in the image detector by image correction in image processing, and therefore point defects do not present a major problem. However, line defects, including leakage of storage capacitor lines, scan lines, and data lines, are difficult to physically repair by laser repair or the like in the TFT array process, and also are difficult to rectify by image correction. Therefore, line defects are critical defects and are a cause of increased production costs for TFT arrays.
The scan lines, gate electrodes and storage capacitor line in FIG. 21 are all formed to the same metal layer (gate layer). Therefore, patterning defects occur when forming the gate layer, and if conductive material remains between the storage capacitor line and either the scan lines or the gate electrodes, leak may occur between the scan lines and the storage capacitor lines.
Such defects occur more frequently as the resolution of the image detector gets finer.
Explanation will now be given of the problem of detected images.
It is obviously essential to reduce electronic noise of the radiation image detector to improve image quality of detected images in the radiation image detector constructed as described above. The electronic noise is influenced largely by data line noise caused by a line capacitance in the radiation image detector using the active matrix substrate constructed as described above. Accordingly, in order to improve the image quality of detected images, a reduction in the line capacitance of the data lines is needed.
The line capacitance of the data line, represented as Cd_line, may be expressed as follows:Cd_line=Ngate×(Cdgx+Cdcsx+Ctft+Cdp)+Ccom 
Where, Ngate is a number of scan lines crossing with the data line, Cdgx is a capacitance at an intersection of the data line and the scan line, Cdcsx is a capacitance of an intersection of the data line and the storage capacitor line, Ctft is a capacitance of the TFT section between the data line and the TFT switch, Cdp is a coupling capacitance between the data line and the charge collecting electrode and Ccom is a capacitance between the bias electrode and the data line.
Because Com and Cdp are normally small and may be omitted, Cd_line may be expressed as follows:Cd_line=Ngate×(Cdgx+Cdcsx+Ctft)
Here, consider a case when a film of 300 nm thick having 7.5 of dielectric constant is used as the gate insulating film 15. A width of the scan line 101 and the storage capacitor line 102 is 10 μm and a width of the data line 3 is 10 μm. While the capacitance of the TFT section is determined by a channel width and a channel length, it is considered to be 0.01 pF. A number of scan lines crossing with the data line is 1,500. Therefore, because Cdgx=0.0256 pF, Cdcsx=0.0256 pF, Ctft=0.01 pF and Ngate=1500, the line capacitance of the data line, Cd_line, becomes=91.8 pF.
Although it is possible to reduce the above mentioned capacitance of the intersections by thickening the gate insulating film 15, a driving capability of the TFT switch 4 drops inversely proportional to that in such a case. Therefore, it becomes necessary to enlarge the size of the TFT switch 4, increasing its area.
In view of the problems described above, the present invention provides an image detector capable of improving the production yield of TFT arrays, and capable of reducing electronic noise and improving the image quality of detected images.